Size gradients of barreloids in the rat thalamus.

The spatial organization of the anatomical structures along the trigeminal afferent pathway of the rat conserves the topographical order of the receptor sheath: The brainstem barrelettes, thalamic barreloids, and cortical barrels all reflect the arrangement of whiskers across the mystacial pad. Although both the amount of innervation in the mystacial pad and the size of cortical barrels were shown previously to exhibit increasing gradients toward the ventral and caudal whiskers, whether similar gradients existed in the brainstem and thalamus was not known. Here, the authors investigated the size gradients of the barreloids in the ventral posteromedial nucleus of the rat thalamus. Because the angles used to cut the brain were crucial to this study, the optimal cutting angles were determined first for visualization of individual barreloids and of the entire barreloid field. Individual barreloids, arcs, and rows as well as entire barreloid fields were clearly visualized using cytochrome oxidase staining of brain slices that were cut with the optimal cutting angles. For the first five arcs (including straddlers), the length of barreloids increased in the direction of dorsal-to-ventral whiskers and of caudal-to-rostral whiskers. These gradients reveal an inverse relationship between the size of barreloids and whiskers (length and follicle diameter) along arcs and rows. The largest barreloids in the ventral posteromedial nucleus were those that represent whiskers C2-C4, D2-D4, and E2-E4, which are neither the largest nor the most innervated whiskers in the mystacial pad. This implies that the extended representation is not merely a reflection of peripheral innervation biases and probably serves an as yet unknown processing function.